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The Atomic Theory
What is the atom and how has it
changed since its discovery?
Atomic Theory
Do Now
Name a murder mystery movie that you’ve seen?
In that movie who did you originally think was the
killer/thief was?
As the movie when on, did you have to change
your mind about who really was the guilty
Atomic Theory
• Atomic Theory is like a
mystery, that scientist have
been trying to solve for
• But instead of trying to find
who is the guilty party, this
mystery is an attempt to find
what the atom really looks
• Each time new evidence is
discovered the appearance
of the atom changes to
incorporate this new
Atomic Theory
• The Greek Philosopher
Democritus (460 – 370 BC)
thought that if you cut a substance
in half a over and over again,
eventually you would end up with
a particle that could not be divided
• Democritus proposed that this
particle that could not be cut any
more, would be called the atomos.
Which means unable to divide.
Atomic Theory
• According to Democritus all atoms were
small hard particles made of the same
material formed into different shapes and
Atomic Theory
• Democritus thought:
– Matter could only be cut up so many times,
eventually there would be a piece that could
not be cut any smaller.
– All matter is made of these particles that he
called Atomos.
– Atomos were made of the same substance,
but different shapes and sizes made different
Copy into your notes
Using a separate sheet of paper, draw a
model of three atoms according to how
Democritus thought atoms (atomos)
looked like.
Copy into your notes
• Atom: The smallest unit of an element that
maintains the properties of that element.
Atomic Theory
• During the years since Democritus first
theorized the atom, scientist learned more
about atoms through many experiments.
• The model of the atom has been altered
and continues to be changed, to reflect
each new piece of information we learn.
Atomic Theory
• John Dalton, a British Chemist in the late
1700’s, did many experiments with
different substances in order to learn more
about the atom.
• His experiments suggested that elements
combine in certain proportions because
they are made of single atoms
• Explain why the appearance of the atom
has changed over time.
• How did Democritus come up with his
model of the atom?
• What did Democritus’ atomos look like?
Include a picture to help explain your
Atomic Theory
• Dalton published his Theory in
1803 and in it he said that
– All substance are made of atoms,
– All atoms are small particles that cannot be
created, divided, or destroyed
– Atoms of the same element are exactly alike
and atoms of different elements are different
– Atoms can join with other atoms to make
new substances.
Atomic Theory
The Atom are tiny
spheres that were
hard and
Daltons Atom
Small, Indivisible, and indestructible
Atomic Theory
• In 1897 a British Scientist named J.J.
Thomson showed that Dalton was
incorrect about atoms being indivisible.
• There were things that were smaller than
a atom and these things could be
separated from the atom
1) All gas is removed
from the glass tube
2) An invisible beam was
produced when the tube was
connected to a source of
3) Metal Plates
could be charged to
change the path of
the beam.
5) When the plates
were charged the
beam made a
glowing spot here
4) When the plates
were not charged
the beam made a
glowing spot here
Atomic Theory
• Thomson concluded that:
1. the beam was made of particles that
have negative electric charges
2. These negatively charged particles are
present in every kind of atom
3. The atom was divisible.
Copy into your notes
• Electron: a subatomic (smaller than an
atom) particle that has a negative charge.
The negatively charged particles that Thomson
discovered are now called electrons
• What did Daltons model of the atom look like.
Include a picture to
• What did Dalton theory say about atoms?
• How did Thomson’s experiments change the
way atoms look? How did it stay the same?
• What did Thomson’s model of the atom look
Atomic Theory
Named after a
popular desert of the
time, my Plum
Pudding Model had
the electrons
embedded in a soft
atom like fruit in a
Thomson’s Atom
Plum Pudding Model of the atom
Atomic Theory
• In 1909 a student of Thomson’s named Ernest
Rutherford designed an experiment to test
Thomson’s theory.
• Rutherford aimed a beam of, charged particles
at a thin sheet of gold foil.
• Rutherford expected for the heavy alpha
particles to pass right through the thin sheet of
• What he found was really surprising. Every
once in a while one of these particles bounced
right back or were deflected to the side.
Some Particles were
slightly deflected from
the straight path
Most of the particles
pass straight through
the gold foil
An element such as
radium produced the
positively charged alpha
A few particles
straight back
The Lead focuses the
particles in to a small
Atomic Theory
• Of this surprising result Rutherford stated:
– “It was quite the most incredible event that
has ever happened … It was almost as if you
fired a fifteen-inch shell into a piece of tissue
paper and it came back and hit you”
– Imaging throwing a brick at a piece of toilet
paper and it bouncing back at you.
Atomic Theory
• Rutherford theorized
that in order to get
results in which
“Bricks” thrown at
paper fly back at you.
The atom could not
be like a pudding or
• In order to get alpha
particles to bounce
back or get deflected
the atoms of the gold
foil had to be mostly
empty space with a
small dense core.
• When the alpha
particle hit the atoms
core, the particle
bounced back. Most of
the time, however, the
particles go straight
through the foil
encountering only
My atom has a very
small center with the
electrons spinning
around this nucleus
Rutherford’s Model of the Atom
Small core with the electrons occupying the space around it
Copy into your notes
• Nucleus: an atoms central region, which
is made up of protons and neutrons and is
positively charged.
• Electron Cloud: the region around the
nucleus of an atom where electrons are
likely to be found.
Atomic Theory
• The next leap in the hunt for
the atom was thanks to the
work of Niels Bohr in 1913.
• Bohr was a Danish scientist
who worked with Rutherford
and studied the way that
atoms reacted to light.
• Bohr wondered why atoms
emitted different wavelengths
of light when exposed to
different amounts of energy.
Bohr theorized that the electrons
of an atom didn’t just orbit the
nucleus randomly, but that they
followed particular paths called
energy levels.
When exposed to
energy, these
electrons jump from
one energy level to
When the electrons lose the
energy, they would fall back
down to a more stable energy
level. And a specific color of light
would be emitted.
Bohr’s Planetary Model
I call this my planetary model
because I theorized that the
electrons orbit around the
nucleus like planets around
the sun
Modern Atomic Theory
What is the Modern Atomic Theory
What is the Modern Atomic Theory
• Have you ever taken a picture that came
out blurry?
• What happened to make your picture that
• Take a look at the picture on the next slide
and make an inference about what was
happening when the picture was taken?
Modern Atomic Theory
• Many more scientists have added to our
understanding of the atom since Bohr.
• We now “know”, thanks to physicists like
Schrödinger and Heisenberg, that
electrons don’t travel in definite paths as
Bohr suggested.
• Instead electrons are found in regions
around the nucleus called the orbitals
Modern Model of the Atom
These orbitals helped to
explain why, when
bonded to other atoms
the molecules take on
certain shapes
Electrons moving at
nearly the speed of
light around the
nucleus could be
found in at any place
in one of these
orbitals at any instant
Modern Atomic Theory States
1. Elements are composed of atoms, which
are extremely small.
2. All atoms of a given element have the same
chemical properties
3. All atoms of a given element contain the
same number of protons.
4. Compounds are formed by the chemical
combination of two or more different kinds
of atoms.
5. Atoms are the units of chemical change
Atoms are composed of three types of particles:`
Positive charge
Found in the nucleus
Neutral (no)
Found in the nucleus
Negative charge
Found orbiting the
• Using the notes and sketches that you
took to day, make an timeline of the
advances in the search for the atom
– should be in chronological order
– Should include the scientist involved
– Should include a model of the atom
– And evidence for the change from previous